Engagement of the T-cell antigen receptor (TCR) results in protein tyrosine kinase (PTK) activation followed by the subsequent phosphorylation of numerous proteins. Previous studies which focused on identifying substrates of TCR-induced PTK activity led to the cloning and characterization of SLP76. In sight into the function of SLP76 WAS provided by the findings that SLP76 regulates TCR-mediated signals that leads to induction of the IL-2 gene promoter. In order to investigate further the function of SLP76 in T-cell activation, we and others have begun to characterize SLP-76 associated proteins. In this study, we focus our attention on the recently identified SLP76 associated phosphoprotein of 130kDa, SLAP130. Sequence analysis reveals that SLAP130 contains several regions that may mediate its interaction with other molecules although it lacks any known enzymatic activity. Initial experiments show that overexpression of SLAP130 interferes with TCR signaling and can inhibit the ability of SLP76 to augment NFAT activity. Although the mechanism of how SLAP130 modulates T-cell activation remains unclear, our preliminary studies provide support for the hypothesis that SLAP130 may serve as a negative regulator of T-cell activation. To test this hypothesis, initial experiments will involve the characterization of the expression pattern of SLAP130 in hematopoietic tissues and during development and T-cell maturation. In addition, we will investigate which proximal TCR-mediated signals are affected by SLAP130. The next set of experiments will focus on understanding the structure/function relationship of SLAP130. We will identify the tyrosine phosphorylation sites in SLAP130 and will ask which of these are responsible for the interaction between SLAP130 and SLP76. In order to understand better the function of SLAP130, we propose to identify other proteins that associate with SLAP130 and will ask whether these interactions are important in modulating T-cell function. Finally, to address the importance of SLAP130 function in T-cells, we will use a SLAP130 "knock-out" mouse to assess T-cell function in the absence of SLAP130 expression. It is hoped that the information gained from these studies will provide insight into the function of SLAP130 in modulating T-cell signaling.